Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. For example, for Sn and Li source materials, the source material may be heating above its respective melting point and held in a capillary tube formed with an orifice, e.g. nozzle, at one end. When a droplet is required, an electro-actuatable element, e.g. piezoelectric (PZT) material, may be used to squeeze the capillary tube and generate a droplet at or downstream of the nozzle. With this technique, a relatively uniform stream of droplets as small as about 20-30 μm can be obtained.
As used herein, the term “electro-actuatable element” and its derivatives, means a material or structure which undergoes a dimensional change when subjected to a voltage, electric field, magnetic field, or combinations thereof and includes but is not limited to piezoelectric materials, electrostrictive materials and magnetostrictive materials. Typically, electro-actuatable elements operate efficiently and dependably within and range of temperatures, with some PZT materials having a maximum operational temperature of about 250 degrees Celsius.
Once generated, the droplet may travel, e.g. under the influence of gravity or some other force, and within a vacuum chamber, to an irradiation site where the droplet is irradiated, e.g. by a laser beam. For this process, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber (e.g debris) which can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements. This debris can include heat, high energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of source material. For this reason, it is often desirable to use so-called “mass limited” droplets of source material to reduce or eliminate the formation of debris. The use of “mass limited” droplets also may result in a reduction in source material consumption.
Another factor that must be considered is nozzle clogging. This may be caused by several mechanisms, operating alone or in combination. These can include impurities, e.g. oxides and nitrides, in the molten source material, and/or freezing of the source material. Clogging can disturb the flow of source material through the nozzle, in some cases causing droplets to move along a path that is at an angle to the desired droplet trajectory. Manually accessing the nozzle for the purpose of unclogging it can be expensive, labor intensive and time-consuming. In particular, these systems typically require a rather complicated and time consuming purging and vacuum pump-down of the plasma chamber prior to a re-start after the plasma chamber has been opened. This lengthy process can adversely affect production schedules and decrease the overall efficiency of light sources for which it is typically desirable to operate with little or no downtime.
With the above in mind, Applicants disclose systems and methods for effectively delivering a stream of droplets to a selected location in an EUV light source.